1. Field of the Invention
This present invention relates to coating semiconductor wafers during a chemical vapor deposition ("CVD") process and more specifically to trapping particles contained within a gas which forms the CVD material.
2. Description of the Related Art
One step in manufacturing an integrated circuit involves growing an inorganic layer on a flat silicon wafer. A low temperature method known as chemical vapor deposition may be used in which a gas containing reactive substances such as, for example, silane and oxygen flows into a reactor chamber. The reactive substances absorb or precipitate onto the top surface of the wafer. Then the substances react to form the desired inorganic layer. In order for the reactions to occur, the top surface of the wafer may have to be bombarded by a plasma. This plasma bombardment is typically caused by an electric potential created in the chamber. The plasma aids the reaction because the plasma's kinetic energy is transferred to an activation energy which is required for the reaction.
Gas which contains by-products is removed from the reactor chamber after CVD is performed in order to prevent undesirable reactions from occurring in a later chemical vapor deposition. In one CVD system, deemed Concept One manufactured by Novellus Systems Inc., gas is forced out of the reactor chamber by a vacuum system until the pressure in the reactor chamber falls to approximately 1000 mTorr.
FIG. 1 illustrates a side view of a portion of the Concept One CVD system. Specifically, an outlet conduit 2 for a reactor chamber 1a is presented. The outlet conduit 2 includes an exhaust box 1b through which a gas may pass from the reactor chamber 1a to a throttle valve 3. Downstream of the throttle valve 3 is a four inch gate valve 5. A four inch vacuum manifold 7 is disposed within the outlet conduit 2 downstream of the gate valve 5. Flanges such as four inch flange 4 mount all the objects disposed within the outlet conduit 2 to each other. Sealing gaskets such as o-ring 6 help prevent gas leakage from the outlet conduit 2.
The vacuum manifold 7 generally forces gas out of the reactor chamber 1a before a new gas can pass into the chamber 1a. The throttle valve 3 may maintain a steady pressure in the reactor chamber 1a when the new gas enters the chamber 1a by partially closing. When the throttle valve 3 partially closes, the vacuum manifold 7 cannot draw as much gas per time from the chamber 1a. The gate valve 5 may control the passage of gas into the vacuum manifold from the reactor chamber 1a.
When reacting substances during a chemical vapor deposition to form a layer of a silicon containing compound on a wafer, a powdery silicon by-product also forms. Hereafter the silicon by-product will be referred to as particles or contaminants. If not properly expelled, powdery particles become mixed in the gas when the gas is forced from the reactor chamber 1a. The gate valve 5 is oriented such that its body extends below the outlet conduit 2. Particles are forced by gravity into the bottom of the valve's body as the gas passes through the gate valve 5.
The opening and closing of the gate valve 5 stirs up (or disrupts) the particles, causing them to contaminate the reactor chamber 1a and the vacuum manifold 7. When particles are present in the reactor chamber 1a during a CVD operation, the inorganic layer tends to not grow uniformly across the wafer surface. Defects in the layer create problems when performing the subsequent steps in integrated circuit manufacturing. When particles enter the vacuum manifold 7, they contact turbine vanes located within pumps of the vacuum system. Since the turbine vanes are at a close tolerance with the pump walls, particles and other contaminants can inhibit the operation of the vanes. The presence of particles in the vicinity of the turbine vanes tends to shorten the life of the pumps.
Cleaning the particles from the body of gate valve 5 becomes necessary but is difficult. It requires shutting down the vacuum manifold 7, opening a service port located at the bottom of the gate valve 5, and removing the particles from the inner portion of the gate valve 5. One drawback to this is that the vacuum manifold 7 has dry pumps in which no oil lubricates the turbine vanes within the pumps. Thus, it is difficult to restart the pumps after shutting them down.
It is therefore desirable that a CVD system be devised which reduces the presence of particles and contaminants in vacuum equipment and on wafers. Further, it is desirable that such particles be removed from the system without shutting down the vacuum system pumps. A CVD system with these features would reduce contamination to the reactor chamber and the vacuum system. The natural result of reduced contamination is higher wafer yields and a more reliable wafer product.